Three parts plus seven parts = 10 parts = the whole, so 300/(3+7)=30, (unit quantity), so the number of cats is 3×30=90; number of dogs=7×30=210.
Total number of animals = 90+210=300; of these 3/10=0.3 are cats and 7/10=0.7 are dogs, the ratio is:
cats:dogs=0.3:0.7=3:7.